Some of the Intel CPUs have AES acceleration.
Sure, it will help.
Is that due to the Ethernet itself?
No, I doubt it. A single 10Gb Ethernet adapter has plenty of bandwidth to cover your needs and given you have no switch in between means you should have very low latency. We would need to go through the many levels of the stack to figure out where the bottleneck is and also understand what your goals are for performance to see if they are even reasonable. What method are you using to transfer the data over your network? Is it SMB/CIFS, iSCSI, SSH, SCP, FTP, SFTP, rsync, etc? Each of these have a different performance characteristic. How are your drives configured in their pool of storage? Is there some kind of RAID with parity happening (raid5/raid6/raid50/60)? What is the underlying filesystem type (etx4, btrfs, zfs, xfs, etc)? Is the configuration such that you're limited to the speed of a single spindle? What is the performance of the source of your data? Is it capable of reading as fast as you want?
My point is, just throwing a faster CPU at this may having minimal to no performance impact.